The present invention relates to a semiconductor device, and more particularly to a configuration of a resistor region in a high voltage monolithic IC (Integrated Circuit).
A resistor in a bipolar or MOS-type monolithic IC is constructed by an elongated resistor region of one conductivity type formed in a semiconductor region of opposite conductivity type, and electrodes are provided on both ends of the resistor region. The value of the resistor is determined by the sheet resistance, and by the width and length of the resistor region. In order to decrease the number of manufacturing steps, the doping of impurities for forming the resistor region is usually effected simultaneously with the doping of impurities for forming regions of transistors or other elements. In such case, the sheet resistance of the resistor region cannot be determined independently, and therefore a desired resistance value is determined by the width and length of the resistor region.
In a pattern layout of an integrated circuit including resistors and other circuit elements, the resistor region is often defined not in a straight shape but in a bent or angled shape. This is particularly true where the length of the resistor region is required to be long for a high resistance. Furthermore, the electrode contact regions at both ends of the resistor region are formed in a square shape that is wider than the resistor region. The bent portion of the resistor region necessarily has corners and the electrode contact regions at both ends have four corners, respectively.
The resistor region is isolated from the semiconductor region by applying such a potential to the semiconductor layer that a PN-junction formed between the resistor region and the semiconductor region is reverse-biased. Where the semiconductor region is of N-type, a positive potential is applied to the semiconductor region, whereas a negative potential is applied to the semiconductor region where it is of P-type. In order that the resistor region be isolated from the semiconductor region regardless of the signal voltage applied to the resistor region, the highest potential or the lowest potential (normally a power supply voltage) used in the monolithic IC is applied to the semiconductor region depending upon its conductivity type. Accordingly, when a voltage of the opposite polarity to the power supply voltage applied to the semiconductor region or a voltage near the opposite polarity power supply voltage is applied to one end of the resistor region, an intense reverse bias voltage is established between the resistor region and the semiconductor region. Further, in a monolithic IC, such as a high power amplifier, supplied with a power supply voltage of 100 V or higher, the reverse-bias voltage between the resistor region and the semiconductor region becomes more intense.
In the PN-junction formed between the resistor region and the semiconductor region, a straight PN-junction portion as viewed in a plan would not be subjected to an avalanche breakdown even upon the application of an intense reverse-bias voltage. In other words, it has a high breakdown voltage. However, at the four sharp, convex corners of the electrode contact regions of the resistor region and at the similarly sharp corner in the bent portion of the resistor region, the electric field is concentrated because the space charge region of the PN-junction is very thin there. The electric field concentration at these convex corners is accentuated as the radius of curvature of the corners becomes less. For this reason, an avalanche breakdown will occur first at the convex corners. Consequently, the breakdown voltage of the resistor region is determined by the breakdown voltage at the convex corners, which is a limitation in a high voltage monolithic IC.
It should be noted that the breakdown voltage at the inside concave corner is much higher than that at the outside convex corner, and may be higher than that at the straight portion of a PN-junction. This is because the space charge region is thicker there and the electric field is not concentrated. Both concave corner and convex corners are associated with a pair of straight PN-junctions. The space charge region usually extends laterally out into the semiconductor region in which the resistor region is formed. The space charge regions of a pair of straight PN-junctions associated with a concave corner merge with each other to form a thick space charge region at the corner, whereby the electric field is not very concentrated. In contrast, the space charge regions of a pair of straight PN-junctions associated with a convex corner barely contact each other, and therefore the space charge region at such a corner is very thin. Accordingly, the electric field is highly concentrated at a convex corner, resulting in a decreased breakdown voltage.
For the purpose of preventing such lowering of the breakdown voltage, various approaches have been proposed. However, even with any of the proposed approaches, the breakdown voltage cannot be enhanced to a satisfactory extent, or else other shortcomings arise such as an increase in the manufacturing steps and the enlargement of the area occupied by the resistor region.